Ripped abs and a strong torso look great and they make you a better runner (core strength, you understand). The best way to get them? With a stability ball.

When my young nephew, Ben, recently asked me why some distance runners look "all bent over," I explained that even though running is a great lifetime activity, it can lead to muscle imbalances and reduced flexibility.

To counter these tendencies, many runners do routine stretching and strength training. Fine. But these only go so far. You can do ab crunches until you scream and still have underlying muscle imbalance and weakness. The reason? Most strengthening exercises tend to isolate one muscle group at a time. They don't work the "core" muscles of your entire torso.

That's where the stability ball comes in. These large vinyl balls have become a favorite in health clubs everywhere, because they add a new dimension to exercise: instability. To counteract this instability, your body -- while balancing on the ball -- recruits groups of core muscles working in unison.

Proof comes from recent research at the biomechanics lab at San Diego State University. Researchers there found that, among other benefits, stability balls were especially effective at working the front and side abdominals in combination. Which is exactly the kind of core strengthening that'll improve your running. And tone those abs.

The following eight stability-ball exercises are designed to improve flexibility, balance, and coordination as well as core strength. They'll also help stabilize your spine to prevent injury to the lower back and hips.

Do these exercises on a day you cross-train, or as a warmup for running, or (my favorite) as a cooldown. Start by doing five repetitions of each exercise, gradually working up to as many as 30 (15 for the Body Arch).
Sit on the ball and walk your legs forward while leaning back until the ball is under your lower back. Place your hands on your shoulders or behind your head. Curl your upper body forward in a crunch motion, then return. You can alternate left and right curls to target the side muscles (obliques). As you get stronger, challenge the obliques more by moving your feet closer together.

Start in a deep squat with the ball against your back. Slowly reach your arms over your head as you extend your legs. Let the ball roll along your spine as you stretch back to form a comfortable arch. Hold for 1 second, then return slowly to the starting position. Repeat.

Benefit: Works the hip flexors, abdominals, and pectorals to increase chest capacity and flexibility in the front of your body so you can run more comfortably with an upright posture.

Lie facedown with the front of your hips on the ball and toes on the floor. Relax your arms, letting your hands touch the floor. Slowly raise your chest and shoulders upward, while spreading your arms up and out to the side and tightening your buttocks. Pause and return to the starting position.

Benefit: Strengthens the muscles in the mid and upper back to stabilize your trunk and complement the front-of-body strength and flexibility gained in the Body Arch.

Lie facedown with your thighs on the ball and hands on the floor, your arms perpendicular to your body. Slowly tuck your legs toward your arms by flexing the hips and knees, letting the ball roll toward your shins. Pause, reverse the motion, and return to the starting position.

Benefit: Strengthens the hip flexors and stretches the lower back and buttocks to make you less injury-prone in those areas. Also increases your range of motion, which helps open up your running stride.

Start in the same position as the Body Tuck, with arms extended to the floor. Keeping your body straight, bend your elbows to lower your chin to the floor. Pause and return by straightening your elbows. (You can make this exercise easier by locating the ball closer to your hips, or harder by moving it toward your feet.)

Lie on your back on the floor with your knees bent and feet resting on the ball. Press your feet into the ball and lift your hips until your body is completely straight from head to foot. While balancing one foot on the top of the ball, bring the opposite knee toward your chest, then return. Bring the other knee toward your chest, then return. Finally, return to starting position on the floor.

Lie on your back on the floor with your knees bent at about 90 degrees and the ball resting between your feet. Extend your arms above your head. Squeeze the ball between your feet as you lift it while doing an abdominal crunch, reaching for the ball with extended arms. Take the ball in your hands and lie back flat with arms and ball above your head. Pause, then reverse the motion by grabbing the ball with your legs.

Stand with your back about 3 feet from the wall, feet pointed forward about shoulder-width apart. Place the ball between your lower back and the wall. Slowly lower yourself as you bend your hips and knees, allowing the ball to support your back as it rolls toward your shoulders. Don't exceed a 90-degree bend at the knees. Return to the starting position.

Benefit: Strengthens the quadriceps muscle group, which will help you on long runs and hilly courses, and help prevent knee injury.

When purchasing a stability ball, refer to the chart below to choose the right size for your height. You can buy "health-club quality" stability balls from Spri Products through its Web site, www.spriproducts.com or by calling (800) 222-7774. The balls cost $22 and up. Or check out other brands at your local sporting goods store.